Conventionally, a two piece can which is produced by integrally joining a can body with a can bottom has been popularly used for beverage cans and food cans. In general, these two piece cans are made of a substrate which is produced by laminating a resin film on a metallic sheet such as a steel sheet or an aluminum sheet. Namely, a portion of the two piece can where the can body and the can bottom are integrally merged is formed by applying a drawing, an ironing or a stretching to the laminate plate.
In such a forming, a denting is liable to occur at the bottom portion of the can so that the can bottom is required to have the enough denting resistance (an index that shows the resistance to the occurrence of crack to the resin film when the denting occurs at the can ). On the other hand, the can body portion is required to have a resistance to peeling off since the resin film is liable to be peeled off during the forming such as drawing.
For improving the denting resistance, it has been proposed to use bi-axially elongated and crystal oriented resin film. However, such a crystal oriented resin film tends to be peeled off easily at the time of drawing or ironing. Accordingly, it is desirable to use the laminated crystal oriented film at the bottom portion which is liable to be dented, while it is desirable to use the laminated film which has no crystal orientation at the can body portion.
As a device which meets the above-mentioned requirements, the applicant of the present invention has proposed a device 100 shown in FIG. 6 of producing a laminate plate adapted for the manufacturing of cans which are integrally provided with can bottom, which is disclosed in Japanese laid-open publication HEI 7-195651.
In operation of the device, a metallic sheet 101 is heated by a plurality of heating rolls 104 up to a temperature which is more than a temperature necessary for adhering of a resin film 102 but below a temperature lowring the degree of the crystal orientation.
Subsequently, while being applied a suitable tension by a tension roll 110, the above-mentioned metallic sheet 101 is made come into contact with a thermal transfer roll 105 which is a final heating roll. The thermal transfer roll 105 has a sufficiently high surface temperature to reduce the degree of crystal orientation of the resin film 102 and is provided with a plurality of indented portions 106 as shown in FIG. 7.
Portions of the metallic sheet which come into contact with the surface of the thermal transfer roll 105 is heated to the temperature which reduces the degree of crystal orientation of the resin film, while portions of the metallic sheet which face the indented portions 106 are not heated so that the portions are held at a temperature which does not reduce the degree of crystal orientation.
Thereafter, when the resin film 102 is adhered to the metallic sheet by means of a laminating roll 107 to produce the laminate plate 103, the resin film 102 with a reduced degree of crystal orientation is adhered to the portions of the metallic sheet which comes into contact with the surface of the thermal transfer roll 105 while the resin film 102 where the degree of crystal orientation is held unchanged is adhered to the portions which face the indented portions. The laminating roll 107 is pressed by a back-up roll 111 so as to tightfittingly adhere the resin film 102 to the metallic sheet 101.
A marking device 112 is provided for providing given marks on the laminate plate 103 to identify blanking positions which become necessary in subsequent blanking operation.
In FIG. 8, blanks 126 which are taken out by blanking a laminate plate 103 produced by the production device 100 are shown in the imaginary line, wherein the the laminate plate 103 is punched out by a press. The portions 125 which face the indented portions 106 of the thermal transfer roll define the center of each punching. These blanks 126 are subjected to a press forming such as a deep drawing so as to obtain can stock material which is composed of the can body and the can bottom forming an integral unit, wherein the portion 125 which face the indented portion 106 of the thermal transfer roll is to be formed into the can bottom and a remaining annular or doughtnut-shaped portion is to be formed into the can body.
With such a production device 100, since a high temperature is given to the portions of the metallic sheet 101 other than the above-mentioned doughtnut-shaped portions, which are, the portions facing the indented portions 106, the adhering strength of the portions of the resin film which correspond to the can body is increased so that the resin film on the can body can withstand a heavy forming applied to the resin film at the time of press forming such as deep drawing.
Although the degree of crystal orientation of the resin film 102 on the can body is lowered, the degree of crystal orientation can be increased again by a heavy forming which is applied to the resin film 102 at the time of press forming such as deep drawing.
On the other hand, the resin film having a high crystal orientation is adhered to the can bottom so that the can stock material which is produced as a final product has a high degree of crystal orientation and accordingly a sufficient denting resistance at the can body as well as at the can bottom.
However, when practically producing the laminate plate for two piece cans with the above-mentioned thermal transfer roll 105, the metal sheet exhibits a temperature difference between the indented portions and the remaining doughtnut-shaped portions on this thermal transfer roll 105 so that the metal sheet extends or shrinks due to a thermal strain. Furthermore, a frictional force between the corners of indented forming portions and the metal sheet which extends or shrinks on this thermal transfer roll is increased. Due to these causes, the metal sheet takes ring-shaped scratches having the same diameter as that of the indented portion at the places where the metal sheet comes into contact with the indented portions on the thermal transfer roll 105. Each scratch has a width of several tens .mu.m and a depth of 1 to 3 .mu.m.
When the resin film is laminated on the metal sheet thereafter, these ring-shaped scratches become a major cause of the occurrence of bubbles between the metal sheet and the resin film. When can containers are manufactured using the laminate plate for two-piece can use in which bubbles are formed as a can stock material, a corrosion resistance of the laminate plate is drastically decreased at peripheries of the bubble formed portions.
The above-mentioned ring-shaped scratches can be removed by decreasing the frictional force between the metal sheet and the thermal transfer roll. Accordingly, the applicant of the present application has made efforts to resolve the occurrence of the ring-shaped scratches by applying a rounding working to the corners of the indented portions and increasing the precision of the finish of the working. This method has succeeded in reducing the occurrence of the ring-shaped scratches and making the size of the ring-shaped scratches smaller to some extent. With this method, however, it has been proved impossible to restrict the degree and size of the ring-shaped scratches to an extent that they fall in an industrially allowable range, namely, at a degree where no bubble is formed between the metal sheet and the resin film of the laminate plate for manufacturing cans having the can body and the can bottom as an integral unit.
The present invention has been made in view of the above and it is an object of the present invention to provide a thermal transfer roll which can prevent the occurrence of the above-mentioned ring-shaped scratches at indented portions and can give a desired temperature distribution to the metal sheet. It is another object of the present invention to provide a method of heating a metal sheet by means of the thermal transfer roll and a manufacturing apparatus for laminate plate provided with the thermal transfer roll.